Contrary to popular opinion, the best scientific debates are not on Twitter. Instead, they’re found in special sections of certain journals where teams of scientists are invited to present contrasting views on hot-button issues, then rebut each others’ takes in a pre-determined (and generally civilized) format. This is where you get a window into what topics are currently roiling the waters in labs around the world.

A few years ago, in the Journal of Physiology’s CrossTalk section, researchers debated the following proposition: “Heat acclimation does/does not improve performance in a cool condition.” This is a question that (as I explained in an Outside article last summer) has obsessed physiologists, coaches, and athletes since a bombshell 2010 study from the University of Oregon found that 10 days of training in 104 degrees Fahrenheit boosted cyclists’ VO2max by 5 percent—even when the subjects were later tested in cool temperatures. Suddenly heat training was being hailed as the poor man’s altitude training. But was the result legit, or just a fluke?

Unlike the way things usually go on Twitter, the academic version of this debate shied away from the extremes. Both sides presented evidence supporting their view, but also acknowledged evidence that seemingly contradicted it. More data, they both agreed, was needed. On that note, a new study in the European Journal of Applied Physiology, from a team of researchers in the United Kingdom led by Mark Waldron of Swansea University, offers some interesting new data for the debate.

One potential source of confusion in previous studies is when you measure the outcome. Should you expect a performance boost within days of starting your heat training? Right after you finish heat training? Or is there some sort of delay before it kicks in? To find out, the researchers decided to test fitness adaptations during a 10-day heat adaptation protocol (on days 5 and 10), and after the heat adaptation was finished (on days 1, 2, 3, 4, 5, and 10).

The study involved 22 cyclists (all male, alas), all of whom were serious amateur cyclists training an average of 14 hours a week and competing regularly. The adaptation protocol was 10 consecutive days of cycling in the lab for 60 minutes at an intensity equal to 50 percent of their VO2max, with half of them in the heat group at a room temperature of 100.4F (38 degrees Celsius) and the other half in a control group at 68 F (20 C). They also continued with their normal training outside the lab, subtracting their lab rides to maintain roughly the same training volume as usual. The outcome measure on the test days was VO2max, a marker of aerobic fitness that has a reasonably good correlation with race performance, tested at 68 F (20 C).

If you looked at the data right after the heat adaptation period, or even a couple of days later, you’d conclude that it makes you worse. The VO2max readings were lower. But three days after the heat adaptation, VO2max readingsstarted to climb, and four days afterwards, they peaked at 4.9 percent higher than baseline, strikingly similar to the 2010 Oregon study. The control group, meanwhile, hardly saw any change. Here’s what the average VO2max values looked like (white squares are the control group, black circles are the heat-adapted group):

(Photo: European Journal of Applied Physiology)

The control group’s values stay reasonably constant from the baseline test (labeledBL) through the two tests during heat adaptation (HA 5 and 10) and the six tests after the heat adaptation (Post 1 to 10). The heat adapted group, in contrast, shows a pronounced decrease during the adaptation period followed by a big rebound after it finishes.

This delayed response is puzzling, because many of the changes associated with heat adaptation happen pretty quickly. For example, here’s what the average heart rate during the 60-minute rides looked like during the 10 days of heat adaptation:

(Photo: European Journal of Applied Physiology)

In this case, positive changes (i.e. a lower heart rate) start happening pretty much right away, and continue steadily throughout the protocol. This suggests the cyclists are improving their cardiac efficiency, meaning they’re getting better at delivering oxygen-rich blood to their muscles with less effort. But that can’t be what leads to improved VO2max, because the two changes happen at different times.

The heart rate data is significant for another reason. All the cyclists in both groups were exercising at the same relative intensity, 50 percent of VO2max. For the control group, that resulted in an average heart rate of a little over 140 beats per minute. For the poor suckers in the heat group, they were initially averaging 170 beats per minute, and even after 10 days were still close to 160 beats per minute. In other words, they were training harder. Could that explain why they saw an improvement in VO2max? It can’t be dismissed—but it’s probably not the whole story. These cyclists were pretty serious, already training for 14 hours a week, so it’s very unlikelythat they’d see a massive 5 percent jump in VO2max just by training a little harder.

Another point the researchers highlight is individual variation. Only one of the 12 heat-adapted athletes failed to improve his VO2max, which is pretty impressive. But the exact timing of the peak was slightly different for each athlete. If you simply take the difference between baseline VO2max and the highest value recorded during the study (instead of the average on a specific day like the fourth day after the heat training protocol finished), the average improvement was 7.1 percent, which is even more remarkable. That’s not really a valid metric, because you’re cherry-picking the highest value from a series of repeated tests, but it is a reminder that four days isn’t necessarily a magic number for everyone. It’s just an average.

In the end, these results nudge the pendulum a little more toward the idea that heat adaptation really can boost your performance even when you’re not competing in hot weather. Even better, they suggest that the biggest benefits come after the heat adaptation is done, which makes it a little easier to work out the logistics of heat training and ensure you have at least a few days, and perhaps even a week, to recover from the heat before competing. I’d still be unlikely to bother with this for a race where I know the weather will be cool. But if there’s a chance it might be hot, then heat adaptation definitely makes sense—and knowing that there’s a chance your heat training will pay off even if the temperatures stay low is a nice little confidence-booster to have in your back pocket.

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